1. Field of the Invention
This invention relates to MOS integrated circuits of the type having a substrate bias, in general, and to a circuit for regulating the substrate bias voltage in a MOS/LSI device, in particular.
2. Prior Art
With the advent of MOS/LSI devices, many advances have been achieved in the electronics industry. These advances are permitted because the MOS/LSI circuits permit many more functions and operations to be performed by a very small electronic network. The electronic network or circuit is, typically, provided by establishing a large number of semiconductor circuits on a single substrate. This is highly advantageous over the now outdated approach wherein discrete components were connected together.
However, other problems have been observed and, for the most part, overcome. For example, it was necessary to perfect the techniques of making the circuitry. It was also necessary to eliminate any cross-talk between channels or circuits on a single substrate. Likewise, it was necessary to overcome problems of heat sinking or other thermal control in the substrate. As noted, for the most part, these problems have been overcome.
However, another problem which is continuously being examined and evaluated is the problem of biasing the common substrate. That is, the substrate bias (i.e., voltage or charge) can have a very pronounced effect on the operation of the circuits on the substrate. For example, the substrate bias can be used to control the threshold voltage on either N-channel or P-channel type devices. As the state of the art advances substrate bias plays an increasingly important role in controlling the operation of other circuits on the substrate and in reducing some parasitic effects, e.g., diode capacitance. Moreover, the substrate bias can vary, inter alia, as a function of the supply voltage, as a function of noise coupled from other circuits, or as a function of the temperature of the substrate. Consequently, these variations in substrate bias can cause variations in the operation of the MOS/LSI device. Consequently, it is highly desirable to control this substrate bias.
However, it is also highly desirable to reduce the number of supply sources which are required for the MOS/LSI device. That is, it is highly undesirable to be required to produce (or supply) a supply voltage which is directed only to the substrate.
In the past, substrate bias circuits have been designed which can be placed right on the substrate of the MOS/LSI device along with other circuitry. However, the devices which are known in the art are generally of a single polarity control. That is, the known devices can control the substrate bias only by preventing it from exceeding a certain value in a certain direction. If the substrate bias exceeds a value in the opposite direction or sense, then the known control devices have little or no effect. However, it is evident that this type of two-way or dual control is highly desirable.